270 General Biology 



fluids of the earthworm. The proteins are changed into peptones, the 

 carbohydrates into a sugar compound, and the fats are divided into 

 glycerin and fatty acids. 



After this process has taken place, the food is ready for absorption. 

 This takes place through the wall of the intestine by osmosis, assisted 

 by an amoeboid activity of some of the epithelial cells. 



It will be remembered from our study of the frog that all parts of 

 a living- organism must be nourished. The food absorbed is now taken 

 into the circulation and made an actual part of the blood. As there are 

 no blood vessels in some parts of the earthworm, some of the absorbed 

 food is also taken into the coelomic fluid so as to bathe the bloodless 

 areas. 



THE CIRCULATORY SYSTEM 



The blood of the earthworm, unlike that of man, is actually red, 

 while the corpuscles are colorless. In man the blood-liquid is colorless, 

 and the corpuscles floating about in the blood-plasma are red. This 

 means that the pigment haemoglobin ( ) is within 



the corpuscle in man and the higher animals, while it is in solution in 

 earthworms. 



The following table mentions most of the important longitudinal 

 blood-vessels (Fig. 167) : 



(1) The dorsal or supra-intestinal, running along the dorsal surface 

 of the alimentary canal, from the posterior end of the body to the 

 pharynx. It then divides into many small branches. 



(2) The ventral or sub-intestinal trunk, lying just beneath the 

 alimentary canal. It also extends from the posterior end of the body 

 to the pharynx where it divides into many small branches. 



(3) The sub-neural trunk, as its name implies, passes along under 

 the ventral nerve cord the entire length of the body. 



(4) A pair of lateral-neural trunks (smaller than those above) 

 lying, one on each side of the ventral nerve cord. 



As in the frog and all other vertebrates, paired arteries, veins, and 

 nerves, pass toward and away from the spinal cord between the various 

 vertebrae, so in each segment of the earthworm tiny branches of the 

 dorsal and ventral trunks, called parietal ( ) branches, 



pass along the various septa dividing the somites, and connect with the 

 body wall, where they split into fine branching capillaries supplying 

 and draining the dermal musculature and epithelium. 



Capillaries from the dorsal branch also supply the digestive tract, 

 while in the anterior region two lateral vessels supply the reproductive 

 organs. 



It will be remembered that in the study of the frog, the circulatory 

 system began with a three-chambered heart. In the earthworm there 

 is no separate and distinct organ such as the heart. In its place there 



